In the jewelry trade the main competitor to platinum, in terms of advantageous physical and chemical properties, is white gold. Gold is not only the name of a precious metal; it is also its color. In addition, in order to comply with Quality Mark Standards and Trade Descriptions Acts in various parts of the world, there are strict limits on what may be described as standard platinum. The British Hallmarking Act, for instance, defines hallmarkable platinum as an alloy or mixture with a composition of not less than 95% of the element platinum, with no negative tolerance. In some countries, a small proportion of the ‘platinum’ may actually consist of platinum group metals. Thus, “standard platinum” can vary in its properties, which properties are normally a function of the composition of pure platinum, including color. Of course, in jewelry applications, a subjective property, such as color, can be important in a commercial context.
Recently there has been increased interest in defining what qualifies as white gold and a code of practice is under international discussion. Several white metals may bleach the ‘yellow’ color from gold but the resulting alloy must also meet Quality Mark Standards. It is generally impractical to make an acceptable white gold above 18 carat, while it is easier at 14 carat and easier still at 9 carat. Eighteen carat white gold is probably the strongest competitor to platinum and so the comparative color of white gold is of considerable interest. Some of the 18 carat white golds have at least a slight yellow tinge. The code of practice relies on a color analyzer under standardized conditions and calibrated against a standard white tile surface to determine the spectrum of white color light components. For white gold, they are then converted to a ‘yellowness index;’ a high value being evidently yellow, and a low, or near-zero, value being neutral white. The values obtained also depend on the reflectance of the surface which, in turn, depends on texture.
Another important factor in the utility of white gold is the hardness value. The harder the material, the easier it is to polish and the better the polish holds with wear. However, one skilled in the art would realize that often a harder material will prove more difficult to work. Hard materials tend to be brittle and will break or shatter when bent or flexed. Therefore, a platinum alloy having a high tensile strength, while maintaining a reasonable working level of ductility would be highly desirable. Ductility is reported as the percent elongation to break. Typically, for a material to be able to fold in half (double back on itself) repeatedly, a percent elongation to break of 50% or greater is needed.
Currently, there is still a need in the art for a platinum alloy that can compete with white gold.